Orthodontic treatment of improperly positioned teeth involves the application of mechanical forces to urge the teeth of one or both dental arches into an alignment which provides correct occlusion and is cosmetically attractive. Most techniques use so-called orthodontic brackets which are small slotted metal or ceramic bodies shaped for direct cemented attachment to the front or rear surfaces of teeth, or alternatively for attachment to bands which are fitted over and cemented to the teeth.
Most orthodontic brackets in current use are of an "edgewise" style as invented by Edward Angle in the 1920's. An edgewise bracket has a generally mesiodistally extending slot which opens away from the tooth surface on which the bracket is mounted, and is typically rectangular in cross section. A resilient curved archwire is seated in the bracket slot, and the wire is bent or twisted before installation so the resulting restoring force exerted by the seated archwire tends to shift, rotate or tip the associated tooth into a corrected position.
The archwire must be somehow secured in the bracket slot to resist dislodging forces as imposed, for example, during brushing of teeth or chewing of food, or by the restoring force of the archwire itself. The bracket is formed with oppositely extending tie wings, around which some form of ligature can be fastened to extend over the seated archwire to hold it against movement out of the slot. Historically, small stainless-steel tie-wire ligatures have been used, and the installation and anchoring twisting of these tie wires is time consuming, sometimes uncomfortable for the patient, and requires considerable skill.
An important improvement in ligation was made in the 1960's by Drs. Anderson and Klein, and is described in U.S. Pat. No. 3,530,583, the disclosure of which is incorporated herein by reference. The improvement is a torus or doughnut-shaped flat ring of circular cross section, and made of an elastomeric polymer such as polyurethane which is compatible with the environment of the mouth. The ring is stretched over the opposed tie wings, and extends over and against the seated archwire. The elastomeric ligature is generally easier and quicker to install than a wire ligature, and twisting of the wire ends (along with the risk of long-term tissue irritation) is of course eliminated. Flat toroidal rings of this type, and of noncircular cross section, have also been proposed (e.g., U.S. Pat. No. 3,758,947).
Elastic ligatures nevertheless remain a challenge to install, as they must be angled upwardly or downwardly to be hooked over and behind the first tie wing (or an elastic hook if the bracket is so equipped) and similarly maneuvered to fit over and seat behind the opposed tie wing. Just as with wire ligatures, installation on posterior brackets is particularly difficult, and even the anterior brackets are awkward to engage due to interference with the patient's lips or gum tissue by tweezers or forceps which grip the ring during installation.
The problem is compounded by a periodic need to remove and reinstall the ligatures when an archwire is changed or requires adjustment, or when the ligatures lose elasticity and restoring force, or become discolored. Elastic ligatures have also been made available in different colors which are appealing to younger patients, and sometimes ligatures are changed for "vanity" reasons when a child wants different colors. Every ligature change, for whatever reason, includes significant chair time, possible patient discomfort, and the time and attention of the orthodontist and assistants. It is to the solution of these ongoing problems that the present invention is directed.